They base the ranking on air quality, water supply and other criteria. Unfortunately, Fresno topped the list.
The magazine cites the Central Valley's geography as a major factor. The fact that the Valley traps a lot of pollutants led to Fresno's number one ranking.
The magazine also mentions issues with groundwater due to fertilizers and pesticides. But Fresno is not alone -- San Jose, Riverside and Los Angeles also made the top 20.